Pronoun case for who and whom
21a. Who was the star of last year’s award-winning movie?
21b. Whom was the star of last year’s award-winning movie?
My answer is: 21a
Yes
Right.
You are correct, the correct sentence is 21a, "Who was the star of last year's award-winning movie?" In this sentence, "who" is used as the subject of the sentence.
To determine whether to use "who" or "whom," you need to identify its role in the sentence. If it is the subject, like in this case, you should use "who."
On the other hand, "whom" is used as the object of a verb or preposition. If you were to rewrite this sentence with "whom," it would be: "The star of last year's award-winning movie was whom?" Here, "whom" is the object of the verb "was."
Remember, choosing between "who" and "whom" depends on whether the pronoun is functioning as a subject or an object in the sentence.